Using atom probe tomography, scientists have demonstrated that lithium ions penetrate and become permanently bound within the copper current collector during charging and discharging. This process has a detrimental effect: every loss of lithium reduces the available battery capacity and shortens the lifespan of a cell.
According to the researchers, this effect worsens with the number of charge cycles and also occurs in modern lithium-metal batteries. Their investigations revealed that lithium accumulates at grain boundaries and interfaces of the copper foil as early as the first charge and discharge cycle. After several cycles, the structure of the current collector changes: its surface becomes nanocrystalline and oxidises, creating additional defects. These defects bind further lithium and oxygen beneath the surface while simultaneously promoting the decomposition of the copper current collector.
The researchers’ method enables the three-dimensional visualisation of individual chemical elements with sub-nanometre resolution, allowing them to observe for the first time where lithium is deposited within the copper current collector. According to the study’s authors, these findings are particularly relevant for the development of future battery generations. Lithium-metal batteries and so-called anode-free (‘zero-excess’) batteries are considered promising candidates for achieving significantly higher energy densities than today’s lithium-ion batteries. However, it had previously been assumed that lithium does not interact, or only minimally interacts, with the copper current collector. The newly published results challenge this assumption and suggest that lithium loss to the current collector has been underestimated as a cause of capacity degradation in such cells.
